[IZPACK-1061] - UserInputPanel: Installer crashes on missing 'set' attribute for RuleInputFields

This is a post-fix with a new unit test for the failing non-standard use case:
```xml
<field type="rule" variable="host">
  <spec txt="Hostname or IP address:" id="host.label" layout="O:25:U" set="0:localhost" />
</field>
```

which initialized the (single) GUI input by "0:localhost" instead of the expected text "localhost".